This invention relates to the process and methods of synthesizing 7-acetylenic quinone methides and using said quinone methides to inhibit and retard polymerization of vinylic monomers. As described in U.S. Pat. No. 7,651,635 quinone methides are used to inhibit the polymerization of vinyl aromatic monomers. These monomers undesirably polymerize at various stages of their manufacture, processing, handling, storage, and use. Vinyl aromatic monomers undergo self-initiated polymerization at elevated temperatures even in the absence of polymerization promoters. Thus, undesired thermal polymerization is particularly problematic especially during the purification process of vinyl aromatic monomers and during emergency shutdown conditions. Undesirable polymerization results in product loss because the valuable monomer end product is consumed. Moreover, polymerization reduces production efficiency as polymer gets deposited on process equipment. This fouling of process equipment may require shutting down so the undesired polymer can be physically removed.
To minimize the problem of unwanted polymerization, two categories of compounds have been developed, namely, inhibitors and retarders. Inhibitors effectively prevent polymerization while the monomer purification process is running uninterrupted. As long as the process is continuously running, the inhibitors are continuously injected into the purification tower along with the feed stream. Conversely, when the process is static, inhibitors cannot be added into the purification tower so that said inhibitors are consumed rapidly. In cases of emergency shutdowns when more inhibitor cannot be added, previously added inhibitor will be rapidly consumed and depleted. Without inhibitors, the unwanted polymerization accelerates rapidly thereby causing fouling of equipment, undesirable consumption of the end product monomer and posing a high safety risk due to runaway exothermic polymerization reactions. Although not as effective as inhibitors, retarders slow polymerization rates. During emergency shutdown conditions, retarders are usually not consumed as quickly so they keep the amount of polymer formed low. As a result, a retarder is more reliable than an inhibitor during emergency shutdown conditions.
At first, only retarders such as sulfur, sulfur dioxide, and dinitrophenols (DNP) (including 2,6-dinitrophenol, 2,4-dinitrocresol, and 2-sec-butyl-4,6-dinitrophenol (DNBP)) were used to prevent unwanted polymerization reactions. Later two classes of inhibitors, viz.; dialkylhydroxylamines (including hydroxypropylhydroxylamine (HPHA)) and stable nitroxides free radicals, were used. To get the most out of both inhibitors and retarders, and because of safety concerns in the event of a plant malfunction, inhibitors are often combined with retarders.
Despite their efficacy and low cost, DNP retarders are highly toxic. Another equally significant disadvantage is that these compounds release NOX emissions during incineration. So, there is an urgent need for a replacement for these retarders. A class of compounds less toxic and environmentally more benign substitute retarders for DNP comprises quinone methides. Quinone methides slow the rate of polymer formation under static conditions and do not need to be replenished unlike inhibitors. Quinone methide retarders are not very economical unless they are used in combination with inhibitors.
Examples of quinone methide compounds are in U.S. Pat. No. 4,003,800. These 7-alkyl quinone methide compounds, however, are not stable enough for sustained use in industrial settings especially in the case of emergency shutdown. Stable quinone methides are more desirable. Other applications of quinone methides are found in U.S. Pat. Nos. 5,583,247, and 7,045,647. Previous examples of inhibitor-retarder combinations that use DNP are disclosed in U.S. Pat. Nos. 5,446,220 and 6,024,894. These combinations were found to be more effective than DNP alone.
As taught in U.S. Pat. Nos. 5,750,765, 5,670,692, 6,926,820 and 7,651,635 some quinone methides have proven effective and are a “green” non-toxic inhibitor-retarder combination for use in preventing the premature polymerization of styrene and other vinyl aromatic monomers. US Published Patent Application 2009/0287013 discusses the synthesis of one such quinone methide, a manufacturing method that involves the use of highly toxic cyanide that has to be handled with great care. Therefore there is a clear utility and novelty in effective methods of synthesizing other efficient quinone methides. Equally as important, there is a need for retarders that are as efficient as DNBP but as safe as prior art quinone methide compounds.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists. Any and all patents, patent applications, and other references cited by this application are hereby incorporated by reference in their entirety.